The need for cooling electronic devices such as infrared detectors to cryogenic temperatures is often met by miniature refrigerators operating on the Stirling cycle principle. As is well known, these cryogenic refrigerators or cryocoolers, use a motor driven compressor to impart a cyclical volume variation to a working volume filled with pressurized refrigeration gas. The pressurized refrigeration gas is forced through the working volume to one end of a sealed cylinder called a cold well. A piston-shaped heat exchanger or regenerator is positioned inside the cold well. The regenerator has openings at each end to allow the refrigeration gas to enter and exit the cold well through the regenerator.
The regenerator reciprocates at a 90.degree. phase shift relative to the compressor piston and the refrigeration gas is force to flow through the cold well in alternating directions. The refrigeration gas is thereby forced to flow from the compressor, or warm end, through the regenerator piston and into the cold end of the sealed cold well and then back. As the regenerator reciprocates, the warm end of the cold well which directly receives the refrigeration gas from the compressor becomes much warmer than the ambient. In the opposite end of the cold well, called the expansion space or cold end, the refrigeration gas becomes much colder than the ambient. A device to be cooled is thus mounted adjacent to the expansion space, or cold end of the cold well such that thermal energy from the device to be cooled is passed to the refrigeration gas through a wall of the cold well.
It is a typical problem in the design of cryocooler systems to reduce the heat load of the cold well so that increased cooling power is achieved. The heat load is defined by the amount of thermal energy which must be removed from the cold well cold end in order to maintain the device to be cooled at the required operating temperature. Alternately, the cooling power is defined as the amount of thermal power removed by the refrigeration gas in order to maintain the device to be cooled at the desired temperature. Heat load is typically reduced by proper selection of the cold well materials, by proper structural design and by selection of surface finishes. The heat load of a system can be determined by use of a boil-off test, conducted at room temperature, whereby a cold well is filled with liquid nitrogen, or the like, and the time required to evaporate the liquid nitrogen is measured.
It is known to reduce convective heat load by providing a housing or dewar surrounding the cold well and by evacuating the dewar to very low vacuum pressures, e.g. as low as 5.times.10.sup.-9 torr, thereby surrounding the cold well with a vacuum space. Thus room temperature air surrounding the dewar is prevented from warming the cold well.
It is also known to reduce radiative heat load of the cold well by coating the external surfaces of the cold well as well as the internal surface of the dewar surrounding the cold well with a highly thermally reflective surface finish, e.g. gold, silver or the like.
The more difficult problem of reducing the heat load of the cold well has heretofore been the problem of reducing conductive heat load passing through the walls of the cold well itself. Thermal energy conducted from the compressor end of the cold well toward the cold end of the cold well may account for as much as 70% of the total heat load. Temperature gradients between the compressor end and the cold end may reach as much as 270.degree. C.
It is known to reduce the cross sectional area of the cold well walls to reduce the conductive heat load. Thin walled cold wells with cylindrical cross-section have been used in the prior art to minimize cross-sectional area. Uniform thickness cold well walls of approximately 0.005 inches are used in the prior art, however, use of even thinner walls reduces the structural integrity of the cold well which could rupture due internal pressures or could cause cyclic movement of the cold end as the working volume of the expansion space varies with each regenerator cycle. Such movement of the cold end is undesirable in optical systems since the lateral or bending motion causes an effective increase in the blur spot thereby reducing system resolution (MTF) and pointing accuracy.
It is also known to use a cold well with a cylindrical cross-section but having a non-uniform wall thickness, e.g. tapering from a first wall thickness at the compressor end to thinner wall thickness at the cold end, to thereby increase thermal resistance near the cold end. This method reduces heat load but requires additional structural elements to maintain the structural integrity of the cold well. The tapered wall cold well is also difficult and expensive to manufacture due to the increased complexity of forming a tapered element, especially a thin walled tapered element.
It is therefore a general problem in the art to improve the performance of cryocooling systems while maintaining substantially similar or a decreased manufacturing cost.
It is a further problem in the art to reduce the heat load of cryocooler systems.
It is a specific problem in the art to reduce the conductive heat load of a cold well while maintaining sufficient structural integrity of the cold well walls for normal operation.